This application claims priority to United Kingdom Patent Application Number 0212354.5, filed May 29, 2002.
The present invention relates to a power drive unit for cargo handling systems, particularly drive units for use in a cargo handling system in the cargo compartment of an aircraft.
Conventional Power Drive Units (PDUs) include rotationally driven rollers which can be raised through an aperture in a deck panel of the cargo compartment frictionally to engage the under-surface of a cargo unit, conventionally a Unit Load Device (ULD) to move the ULD within the cargo compartment. It would be understood however that such PDUs can be used to handle ULDs and other cargo units in cargo handling systems external to the cargo compartment of an aircraft or other vehicle or vessel.
In the accompanying drawings FIG. 1 is a cross-sectional representation of a conventional PDU. The conventional PDU includes a rigid metallic frame 11 pivotally mounted at 12 to a chassis (not shown in FIG. 1) rigidly secured beneath a deck panel of the floor or deck of a cargo compartment. The axis 13 of pivotal movement of the frame 11 relative to the chassis is parallel to, and spaced below, the plane of the deck panel. Adjacent its end remote from the pivot axis 13, the frame 11 rotatably supports a roller assembly 14 for rotation about an axis 15 parallel to the axis 13. The roller assembly is disposed in alignment with an aperture in the deck panel and in a rest position of the frame 11 relative to the deck panel rubber tyred rollers 15a of the roller assembly 14 lie just beneath the plane of an array of Ball Transfer Units (BTUs) carried on the upper surface of the deck panel to provide a low friction support for a ULD on the deck panel. There is provided an arrangement for raising the PDU by pivoting the frame 11 about the axis 13 to raise the periphery of the rollers 15a through the aperture in the deck panel to engage the under-surface of a ULD seated on the BTUs.
Mounted within the frame 11 is an electric drive motor 16 the rotor shaft 17 of which is equipped, at one end of the shaft 17, with an electro-magnetically operable brake assembly 18. The brake assembly 18 when operative brakes the shaft 17 of the motor against rotation.
The opposite end of the shaft 17 from the brake 18 is equipped with a small diameter pinion gear wheel 19 which meshes with the teeth of a large diameter internal gear wheel 21 mounted to the frame 11 for rotation about an axis parallel to the axes of the shaft 17 and roller assembly 14. A shaft 22 extending from the gear wheel 21 and rotatable therewith is formed with a small diameter pinion gear wheel 23 meshing with a larger diameter gear wheel 24 on a shaft 25 mounted to the frame for rotation about an axis co-extensive with the axis of rotation of the shaft 17. A train of gears 43, 44, 45 all rotatable about parallel axes transmit drive from the gear wheel 24 to a shaft 46 carrying the gear wheel 45. The shaft 46 carries the axially aligned rollers 15a for rotation therewith about the axis 15, the shaft 46 being journalled at its opposite axial ends respectively in bearings on the frame 11 for rotation relative thereto, and the gear wheel 45 being disposed adjacent the mid-point of the length of the shaft 46 between the rollers 15a. It will be recognised therefore that when the brake 18 is de-energised to release the shaft 17 and the motor 16 is energised then the motor 16 drives the rollers 15a for rotation in unison about their common rotational axis 15.
The end of the shaft 25 remote from the motor 16 is coupled to an axially co-extensive drive shaft 26 through the intermediary of a torque limiting device 27 and an electro-magnetically operable clutch 28. A small diameter pinion gear wheel 29 on the shaft 26 meshes with a larger diameter gear wheel 31 on a shaft 32 parallel to the shaft 26. A smaller diameter gear wheel 33 on the shaft 32 drives a larger diameter gear wheel 34 driving a co-axial gear wheel 35 meshing with a gear wheel 36 on the end of a cam shaft 37 journalled for rotation in the frame 11. The cam shaft 37 extends the full width of the frame 11 and adjacent its opposite axial ends respectively carries first and second cams 38 which cooperate with fixed cam followers on the chassis of the PDU whereby angular movement of the shaft 37 about its longitudinal axis lifts and lowers the frame 11 about the axis 13 by virtue of the cam action between the cams 38 and the cam followers on the chassis.
The operation of the conventional PDU illustrated in FIG. 1 is as follows. Let us assume firstly that the brake 18 is operative, the clutch 28 and the motor 16 are de-energised, and the cam shaft 37 is in a rotated position such that the frame 11 is collapsed into its rest position below the level of the deck panel. In order to raise the rollers 15a through the aperture in the deck panel to engage a ULD, power is supplied to the motor 16 and at the same time the brake 18 is de-energised so that the shaft 17 is released for rotation. Simultaneously power is applied to the electromagnetic clutch 28 so that the clutch is engaged and rotational movement of the shaft 17 is transmitted through the torque limiter 27 and the engaged clutch 28 to the shaft 26.
Simultaneously rotational movement of the motor shaft 17 is transmitted through the gear train 24, 43, 44, 45 to the rollers 15a to rotate the rollers 15a about their axis 15.
Rotation of the shaft 26 drives the shaft 37 through the step-down gear train 29, 31, 33, 34, 35, 36 interconnecting the shafts 26 and 37. The shaft 37 is thus moved angularly about its longitudinal axis causing the cams 38 to cooperate with the chassis and thus raise the frame 11 relative to the deck panel about the axis 13. The periphery of the rollers 15a is thus caused to project upwardly through the aperture in the deck panel so as frictionally to engage the under-surface of a ULD supported on the deck panel. As the rollers 15a are being rotated by the motor 16 the ULD will be moved relative to the deck panel.
It will be noted that the roller assembly 14 includes a rubber tyred wheel 39 mounted for rotation about the axis 15 and having an outer diameter similar to the outer diameter of the rollers 15a. The wheel 39 engages the under-surface of a ULD at the same time that it is engaged by the rollers 15a. However, the wheel 39 is not driven with the rollers 15a and thus can detect slip between the rollers 15a and ULD since in such a situation the rollers 15a will continue to rotate but the wheel 39 will be stationary, or moving at a different speed, by virtue of its engagement with the ULD. A slip sensor detects any difference in the rotational speeds of the wheel 39 and the rollers 15a and either provides warning of slippage or alternatively de-energises the motor.
When the engagement between the cams 38 and the cam followers reaches its highest point (corresponding to the maximum lift position of the frame 11) the cams 38 engage stops which prevent further rotation of the shaft 37. Thus the gear train and the shaft 26 become stalled since the shaft 37 cannot rotate any further, and the torque limiting device 27 slips so that the motor 16 can continue to operate and to drive the rollers 15a notwithstanding that the shaft 37 is now held against further rotation. It will be recognised that energy is dissipated within the torque limiting device 27 as the device 27 slips throughout the whole of the time that the rollers 15 are rotated in their fully raised position.
In the event that the rollers 15a are subjected to a shock loading in a vertical direction then they can be depressed relative to the deck to accommodate such a shock loading by reverse rotation of the cams 38 and the shaft 37 as permitted by slippage in the torque limiting device 27. Immediately the loading is removed then the rollers will be returned to their fully raised position. This arrangement also accommodates unevenness in the under-surface of a ULD or other cargo unit.
When it is required to reverse the direction of rotation of the rollers 15a in order to drive a ULD in the opposite direction the polarity of the motor 16 is reversed so that the rotor shaft 17 of the motor is rotated in the opposite direction. The effect of this is to reverse the rotational direction of the shaft 37 so that the cams 38 are moved away from their stops lowering the frame 11 to its fully lowered position, and thereafter the cams, which are symmetrical about their rest point, start to raise the frame again by rotation of the cams 38 beyond their rest position. The rollers 15a are of course being rotated in the opposite direction during this movement. Rotation of the shaft 37 ceases when the cams 38 engage their stops with the frame 11 full raised, but with the rollers 15a now rotating in the opposite direction to the previous operation.
If it is desired to brake the motion of a ULD in contact with the rollers 15a the power supply to the motor 16 is broken and the power supply to the brake 18 is re-established to brake the shaft 17 against rotation. Thus as long as the ULD is moving in the direction in which it was driven by the rollers 15a then the cams 38 will remain against their stops, the shaft 37 will not rotate, and the rollers 15a will remain in their raised position but will not be rotated so applying a braking force to a ULD moving relative thereto
In order retract the rollers from their operative position to their rest position below the deck panel the clutch 28 is de-energised so that the shaft 26 can rotate freely irrespective of the shaft 17 being held against rotation. Torsion springs 41 acting on the shaft 37 can now rotate the shaft 37 in the reverse direction moving the cams 38 away from their stops and lowering the frame relative to the chassis. Reverse rotation of the shaft 37 is permitted by rotation of the gear train coupling the shaft 37 to the shaft 26, and the freedom of rotation of the shaft 26 by virtue of de-energisation of the clutch 28.
The conventional PDU suffers from a number of recognised disadvantages. Firstly, power dissipated within the torque limiting device 27 during normal operation of the PDU is wasteful of energy, and generates heat which may be a problem in some environments. Secondly, when it is necessary to reverse the drive provided by the rollers to the ULD the PDU must go through a sequence of being lowered to its rest position and then raised again fully to its operative position before the drive from the PDU to the ULD is reversed. This sequence is often referred to as xe2x80x9clift-lower-liftxe2x80x9d and the time taken to do this can be several seconds leading to significant operator frustration.
An alternative known form of PDU utilizing a differential gear drive mechanism is disclosed in U.S. Pat. No. 5,938,003. Such PDUs are disadvantageous in that they are very complex to manufacture and assemble and thus are expensive to supply and maintain. Moreover the arrangement disclosed in U.S. Pat. No. 5,938,003 is disadvantageous in that the roller braking mechanism 90 is permanently operative and so consumes power and generates heat and wear in normal use; the lifting mechanism, if obstructed during lifting may not assume the fully raised position after the obstruction is removed; and the roller 30 being cantilevered from a bearing at one end only of its support shaft requires the use of heavy duty bearings to ensure a long working life.
It is an object of the present invention to provide a PDU wherein the aforementioned disadvantages are minimised or obviated.
In accordance with the present invention there is provided a power drive unit (PDU) comprising a frame arranged to be mounted in use for raising and lowering movement relative to a supporting structure, a drive motor carried by the frame and having an output shaft, a drive roller assembly carried by said frame for engagement in use with a unit load device (ULD) or the like to be moved by the PDU, a first gear train transmitting drive from said motor output shaft to a drive roller of said drive roller assembly, a rotatable lifting cam assembly carried by said frame and driven in use relative to said frame to lift and lower the frame on said mounting, a second gear train for transmitting drive from said motor output shaft to said lifting cam assembly to operate said lifting cam assembly, clutch means operable to connect and disconnect said motor output shaft to and from said second gear train, a torque limiting device limiting the torque transmitted through said clutch, when said clutch is engaged, to said lifting cam assembly, and a brake mechanism between said torque limiting device and said lifting cam assembly for braking said lifting cam assembly against rotation relative to said frame.
Preferably the power drive unit includes a control system which is arranged to receive control input from an operator and to effect control over operation of said motor, said clutch, and said brake mechanism.
Desirably said control system includes a timer which is arranged so that after the lapse of a pre-determined time period from the point at which the motor is operated to raise the frame, the control system initiates operation of said brake to lock said lifting cam assembly, and disengages said clutch to disconnect said second gear train from said motor output shaft.
Alternatively said control system includes a sensor detecting the fully raised position of the frame and in response thereto signalling the control system to initiate operation of said brake to lock said lifting cam assembly, and disengage said clutch to disconnect said second gear train from said motor output shaft.
Conveniently the control system includes both a timer which is arranged to produce a signal after the lapse of a pre-determined time period from the point at which the motor is operated to raise the frame, and a sensor detecting the fully raised position of the frame and producing a signal responsive thereto, the control system responding to the earliest of, or both, signals to initiate operation of said brake to lock said lifting cam assembly, and disengage said clutch to disconnect said second gear train from said motor output shaft.
Preferably the control system is so arranged that said clutch is disengaged fractionally after engagement of said brake.
Preferably a resiliently compliant mounting system is interposed between said frame and the fixed support structure supporting the power drive unit. Conveniently said power drive unit includes a chassis to which said frame is pivotally mounted, and a resiliently compliant coupling mechanism securing said chassis to the fixed support structure.
Preferably the roller assembly includes first and second axially aligned rollers carried by bearing supports at both axial ends of the assembly.
Preferably there is provided a further brake for braking the motor output shaft, said further brake also being under the control of said control mechanism.